Development and effective utilization of natural gas (methane) resource represent the development direction of the contemporary energy structure and is also one of important ways of ensuring sustainable development and energy greening. In recent years, western developed countries have made breakthrough in the development aspect of shale gas and “combustible ice”, resulting in a “shale gas revolution”. In China, shale gas resources have many types and are distributed centrally relatively. The recoverable resource potential is 25 trillion cubic meters (Qinghai-Tibet region excluded), which is equivalent to that of conventional natural gas for land in China and close to 24 trillion cubic meters of America. The “Twelfth Five-year plan” in China has deployed in the development field of the shale gas to obtain technical breakthrough in several different kinds of shale oil and gas regions and to preliminarily establish productivity with economic benefits.
However, how to efficiently use gaseous hydrocarbon resources (methane) has already become an important link that restricts the development of energy industry of China. It reignites the world-wide interest to convert such abundant resources into fuel and high-added-value chemicals (especially light olefins), and also an important step for improving the energy structure of China. Light olefins, such as ethylene, etc., are very important raw material or intermediates in the chemistry and chemical engineering process. Traditional light olefins (C2-C4) mainly come from the petrifaction process of naphtha cracking, etc., so that the production of the olefins has already become a symbol for measuring the petrochemical production level of one country and region. As petroleum resources are increasingly exhausted, it is a focus of current study to explore a method for making the light olefins in a non-traditional route. Accordingly, some typical substitute routes arise at the historic moment, such as the route of starting from synthetic gas and further converting methanol or dimethyl ether to obtain the light olefins, but this route has complicated process and lower atom economy. To shorten the reaction route, a large number of studies are carried out on direct synthesis of the light olefins through a Fischer-Tropsch route starting from the synthetic gas. However, the above substitute route must consume CO or H2 to remove O in CO, which may inevitably cause C atom utilization rate lower than 50%. In spite of expensive productivity input, heavy CO2 emission and atom utilization rate lower than 50%, the indirect process still occupies a dominant status in application of natural gas industry.
In contrast, direct conversion of natural gas has enormous economic potential and is more environmentally friendly. However, direct conversion of natural gas remains a difficult problem in the chemistry and chemical engineering process. The essential component of natural gas is methane. The bond energy of C—H bond is as high as 434 kJ/mol, while the methane molecule itself almost has no electron affinity. In addition, ionization energy is large and polarization rate is small. Therefore, activation of the C—H bond of methane is regarded as a “holy grail” of chemistry and even chemistry field. Keller and Bhasin has reported activation of the C—H bond of methane under the participation of O2. Their pioneering work ignites the world-wide enthusiasm on studying preparation of ethylene from oxidative coupling of methane under high temperature (>1073K), in which hundreds of catalysis materials are synthesized and tested. The study has reached the peak in the 90s. In the oxidative coupling process, because of the introduction of molecular oxygen (O2), over oxidation of methane and its products are inevitably caused, thereby producing a great number of products with stabler thermodynamics than methane, such as CO2 and H2O and finally causing relatively low C atom utilization efficiency. Due to the bottleneck of the development of new material and raw catalysts, the development of the oxidative coupling process of methane stagnates. So far, new technologies with economic feasibility are rarely reported. A recent study proposes that gas-phase S with weak oxidability substitutes for molecular oxygen O2 to generate an oxidative coupling reaction of methane. At temperature of 1323K (reaction gas: 5% CH4/Ar), an optimal PdS/ZrO2 catalyst can realize 16% of methane conversion; however, C2H4 selectivity is only about 20%, but can produce a great number of CS2 and H2S. The above study reveals that oxygen (or oxidizer) assisted methane activation inevitably causes over oxidation.
Therefore, direct conversion of methane without oxygen (or without oxidizer) is considered to be an ideal activation and conversion route of methane. Under the condition of no oxygen (or no oxidizer), over oxidation of methane or products can be effectively avoided, thereby inhibiting emission of greenhouse gas CO2 and then increasing C atom utilization rate. The challenges of preparing ethylene by direct catalytic conversion of methane are that: 1) the first C—H bond is broken through controllable methane activation; 2) depth dehydrogenation on a catalyst surface is suppressed; 3) generation of greenhouse gas CO2 and carbon deposition is avoided, wherein 1 and 2 are for the catalyst, while 3 is for the reaction process. Over oxidation of products of the aerobic process is inevitable, resulting in inevitable generation of CO2. Only the oxygen-free process can avoid producing CO2, but is easy to produce carbon deposition. Therefore, a study on how to avoid carbon deposition becomes a current focus of the oxygen-free process. The key to solve the carbon deposition problem is to understand a source of carbon deposition. By taking an oxygen-free aromatization process as an example, the carbon deposition mainly comes from: methane is deeply dehydrogenated to generate carbon deposition (“graphite-like carbon deposition”) on the surface of Mo species of the catalyst; in the diffusion process, the products are cyclized and coupled on B acid site of a duct or orifice of a support and zeolite to generate carbon deposition (“polyaromatic carbon deposition”). Therefore, three challenges of preparing ethylene by direct catalytic conversion of methane are design and construction of the catalyst.
In 1993, researchers from Dalian Institute of Chemical Physics (DICP) reported CH4 dehydroaromatization in a continuous flow pattern on the Mo/HZSM-5 catalyst for the first time. At 973K and normal pressure, CH4 conversion is about 6% and aromatics selectivity is greater than 90% (exclusive of carbon deposition in the reaction), forming an important milestone of study on CH4 dehydroaromatization process. In the past decades, the study work of multinational scientists mainly focuses on preparation, development, reaction and deactivation mechanism of the catalyst. Nevertheless, industrial applications are restricted by the rapid carbon deposition and deactivation of the catalyst.
Recently, a composite catalyst prepared by American siluria company (US201241246, US2013165728, US2014121433, CA2837201 and U.S. Pat. No. 8,921,256B29) using a biological template method generates 26% of methane conversion and 52% of ethylene selectivity in the oxidative coupling reaction at 600-650° C. At present, the company is performing pilot production, and is expected to conduct industrialized demonstration in 2017-2018. For preparation of methanol or formaldehyde from selective oxidation of methane, because the oxidation velocity of target products of methanol and formaldehyde is much higher than that of methane as raw material, the reaction selectivity is lower and the products have hardly scale application prospect.
Two patents (application numbers: 201310174960.5 and 201511003407.0) have been applied earlier. These two patents mainly apply for a metal doped silicon-based catalyst so as to realize the process of producing alkene through catalytic conversion of methane by a fixed bed or fluidized bed or mobile bed by placing the catalyst into a reactor. The two methods have the disadvantages of large pressure drop of catalyst bed layer, poor heat conduction of catalyst, large temperature difference of bed layer, harsh preparation condition of catalyst, difficult scaleup, etc.
Therefore, the purpose of the present invention is to lattice-dope active metal or non-metal component into the inner wall of a quartz or silica carbide reactor with a unique shape, or lattice-dope active metal or non-metal component into quartz or silica carbide and coat on the inner wall of the reactor, so that the catalyst and the reactor become a whole. Compared with the above two patents, the method has the following advantages:
1) In the high temperature melting process, internal temperature difference of the catalyst is small, and the repeatability is good. The traditional preparation of catalysts from the laboratory (gram level) to the industrial level (tonnage level) requires many years of repeated stepwise scaleup. The duration of the process is determined by the severity of the preparation conditions of the catalysts. At present, the catalyst needs to undergo a high temperature melting process of 1800-2200° C. The temperature can realize uniform melting for a small number of (gram level) catalysts, but if gram level is scaled up to kilogram level or tonnage level, the internal temperature difference will increase and the repeatability of the catalyst will become poor.
2) For the reaction process, there is no axial or radial temperature difference of the catalyst bed. Because after the catalyst is filled in the reactor, due to poor thermal conductivity of the catalyst, the radial temperature difference of the bed increases (the temperature presents a decreasing trend from the reactor wall to the center). Thus, more heat shall be supplied to reach the reaction temperature at the center of the catalyst bed, resulting in the problems of heat loss and more side reactions at the near wall section (high temperature end).
3) There is no pressure drop in the bed, and the reaction process is more stable.
4) The non-oxidative synthesis of ethylene from methane is a free radical reaction. The catalyst placed on the wall surface provides a larger space for the subsequent gas phase process, which can greatly improve the conversion rate of the methane.
5) It can solve the scaleup problem of catalyst. At present, the pipe diameter of an industrial shell and tube reactor is usually about 50 mm. Meanwhile, the tube diameter of a laboratory microreaction device is about 20 mm. Namely, the optimized catalyst obtained in the laboratory can be easily amplified to 50 mm, so as to avoid the problem of catalyst scaleup.
The process is easier for industrial amplification. At present, the industrialized methane reforming and ethane pyrolysis adopts the shell and tube reactors (no catalyst bed). Namely, the adopted shell and tube reactors are directly replaced with the catalytic reactor of the present invention to realize production of ethylene through methane.